What /6 will fit

As a long block, they'll all work for your '64 but as 805moparkid said, the '68 and later crank has a bigger converter hole so you need to get the conversion bushing to reduce the hole diameter to fit your existing converter button. Naturally, when you swap different years, you're going to run into various casting numbers, brackets, carbs, manifolds, oil pans, mounts, etc, but if you swap those parts from your '64, they will fit on the later long blocks. "81 and newer is a hydrolic cam and uses a different valve cover than the earlier ones so if you get one of those, make sure you get the valve cover with it. Also, some years have a head with a smog pump port that will need to be plugged for the earlier setup.